The invention relates to a device for generating a synthetic signal for testing knock control functions and, more particularly, to a device for generating a synthetic signal for testing knock control functions in engine control devices of motor vehicles.
Electronic control devices are conventionally employed in motor vehicles to control and regulate various engine functions. For example, knock control is performed by means of such control devices as one of the engine functions. "Knocking" or "pinging," which is an uncontrolled form of firing produced by dieseling at an ignition point or ignition angle that is too far advanced, can cause damage to the engine if it occurs too frequently and too violently. For this reason, the ignition point and/or ignition angle is normally set so that it has a safety interval with respect to the so-called knock limit. But an ignition angle that is too far retarded is also undesirable since it results in increased fuel consumption and hence in greater exhaust emissions. Thus, the knock limit is usually detected in engine control devices during operation and the ignition angle is adjusted accordingly.
For knock control, a knock sensor is used that picks up the typical noises that occur during knocking, converts them into electrical signals, and passes the signals to the electronic engine control device. The mounting location of the knock sensor is usually selected so that knocking can be reliably picked up from every cylinder under all conditions. Knock detection for every firing in every cylinder is determined by comparing the knock signal or sensor signal picked up by the knock sensor with a reference level that constantly adjusts automatically to operating conditions. When knocking occurs, the ignition point and/or ignition angle is retarded by a predetermined angle on a cylinder-selective basis. This process is repeated at each firing for every cylinder that is recognized to be knocking. When no further knocking occurs, the ignition point and/or the ignition angle is slowly advanced in small steps until it reaches its characteristic map value. Since the knock limit differs from cylinder to cylinder in an engine, and changes considerably within the operating range, each cylinder has an individual ignition point at the knock limit.
For cylinder-selective knock recognition, a knock window is usually established in the control device for each cylinder and is defined for each cylinder as a function of engine rpm and frequency.
To develop and test such a knock control in electronic control devices in the laboratory, the signal normally supplied by the knock sensor must be realistically simulated. The signal that results in the vehicle and is usually picked up by the knock sensor consists of a noise-like basic noise signal for as long as no knocking occurs, said signal, when knocking does occur, having superimposed on it a knocking signal that resembles a sinusoidal signal and is definitely elevated in amplitude with respect to the basic noise signal level.
For simplification, a synthetic signal has been used heretofore for testing knock control functions, especially in the laboratory, instead of the signals usually picked up by the knock sensor. The synthetic signal, as the basic noise signal, uses only one permanently defined square-wave pulse packet with various permanently specified frequencies and permanently specified low amplitudes, and uses as the knock signal only a square-wave pulse with a permanently set frequency and a permanently set high amplitude. Only a very limited static test of the functional ability of knock recognition and knock control is possible with a simplified synthetic signal of this kind, especially for only one operating point.
The goal of the invention is to improve a device for generating a synthetic signal in such fashion that the synthetic signal is as realistic as possible in comparison to the knock sensor signal that usually appears in engines, and all the functions associated with knock control can be tested without limit in the laboratory, in other words without having an actual engine with a knock sensor and ignition angle retardation.
This goal is achieved according to the present invention by a divide for generating a synthetic signal for testing knock control functions in engine control devices of motor vehicles. A basic noise signal generating unit and a knock signal generating unit provide output signals which can be superimposed to form a total signal. An actuating device is provided for changing the amplitude and/or frequency curve of the basic noise signal, the knock signal (KS), and/or the total signal.
According to the invention, the device has firstly a basic noise signal generating device and a knock signal generating device whose output signals can be combined to form a total level, as well as an actuating device to change the amplitude curve and/or the frequency curve for example of the frequency spectrum, the basic noise signal, the knock signal, and/or the total signal.
The basic noise signal is preferably generated in the form of a noise signal with a low average amplitude and the knock signal is preferably generated in the form of a sinusoidal signal with an elevated average amplitude. The variability of the amplitude and/or frequency or of the amplitude and/or frequency curve, especially of the knock signal, is considerable, as are those of the basic noise signal and/or the total signal. Preferably, the amplitude-or amplitude curve can be changed manually and the frequency or frequency curve can be changed automatically as a function of any operating parameters.
Since usually the test of knock control functions also includes a test of the knock sensor in general, i.e. independently of the occurrence of a knock, preferably the amplitude not only of the knock signal but also of the basic noise signal can be changed.
Since the engine control device usually performs acquisition and evaluation of the knock sensor signal only within a defined so-called "knock window" for each cylinder, the knock signal is generated only for the duration of the knock window of each cylinder, for example.
With this device according to the invention, a test of knock control functions is possible that can be adapted to different operating conditions or types of engines. Different control devices for different engines can therefore be tested as well. The control devices for different engines can differ for example in the threshold value definitions relative to the amplitude of the knock sensor signal to recognize knocking or with regard to the design-related knock signal frequencies. In addition, the synthetic signal can be adjusted by the device according to the invention depending on the way in which the knock sensor signal is evaluated in the control device for engines. For example, there are control devices that compare the reference level as a threshold value for knock detection with maximum amplitude values or with average amplitude values. Accordingly, the curve of the knock signal amplitude can be adjusted. In summary, the device according to the invention provides a very flexible system for testing knock control functions independently of the type of engine or the method of evaluating the control devices.
In one advantageous embodiment according to the invention, the change in amplitude and/or frequency or in the amplitude and/or frequency curve can occur as a function of a signal that is proportional to engine rpm or as a function of the engine rpm signal itself.
This improvement according to the invention takes into account the fact that in certain engines in particular, the frequency and/or the frequency spectrum of the knock signal changes as a function of engine rpm. For this purpose, control devices are provided for example that switch between various evaluation frequency bandpasses to evaluate the knock signal as a function of engine rpm. In addition, an engine-rpm-dependent threshold value setting for detection of knocking is usually chosen in control devices, in other words the higher the engine rpm, the higher the amplitude threshold above which knocking is detected. This improvement according to the invention takes into account the operating parameter with the greatest influence on the occurrence and evaluation of a knock signal.
In another advantageous embodiment according to the invention, the knock signal amplitude and/or frequency can be changed in a cylinder-selective manner.
Since different knock signals can occur in each cylinder under the same operating conditions, so that cylinder-selective knock control is performed in control devices, it is also possible to generate the synthetic signal on a cylinder-selective basis to test the knock control functions. Thus, according to the invention, a separate knock signal can be generated for each cylinder with regard to amplitude and/or frequency. Preferably the cylinder-selective knock signal is generated only within the specified knock window of the respective cylinder. However, it is also possible for example to output the cylinder-selective knock signal from the top dead center point of one cylinder to the top dead center point of the next cylinder.
Thus, the synthetic signal can be optimally adjusted to reality and to the cylinder-selective knock control functions provided.
In another advantageous embodiment according to the invention, the knock signal frequency can be preset within a preset number of crankshaft revolutions or operating cycles.
The knock control functions within the control device act in particular to adjust the ignition angle. The more frequently a knock signal occurs, the more the ignition angle is adjusted. In order to test this preset function, the device is so designed according to the invention that the frequency of the knock signal can be preset and changed,; preferably on a cylinder-selective basis. Another result of this improvement according to the invention is that even more comprehensive testing of knock control functions in the control device is possible.
In yet another advantageous embodiment according to the invention, the beginning of the knock signal can be preset as a function of the signal that is proportional to the engine rpm.
Since the knock windows for evaluating the knock signal begin as a function of the signal that is proportional to the engine rpm, in the device according to the invention the knock signal is preferably also generated in the control devices only when the defined knock window begins. Preferably, the knock signal is generated as realistically as possible for this purpose in the form of an amplitude-modulated sinusoidal signal packet whose amplitude is initially maximal and then decreases steadily. The greater the realism with which the knock signal is generated, the more comprehensively the knock control functions can be tested. Some control devices evaluate only the absolute amplitude, for example the maximum amplitude, to evaluate the knock sensor signal, but other control devices generate the average of the amplitudes over the total signal. In the second case in particular, a realistic knock signal is especially desirable.
The device according to the invention permits a synthetic signal to be generated that can be adapted optimally and flexibly to the requirements imposed by individual control devices.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.